There are a variety of applications for head-mounted display systems. These systems typically include image displays such as micro-flat-panel display devices based on organic light emitting diode (OLED) technology or active-matrix liquid crystal display (AMLCD) technology, for example. The systems also include eyepieces that couple the light from the display devices and possibly light from the environment to the users' eyes.
The head-mounted display systems are applied to the military, in field operations and simulations, medical procedures, and industrial maintenance operations. There are now also opportunities to use head-mounted displays for game consoles, cellular phones, portable video players, and other entertainment and communication devices.
In these head-mounted display systems, simple, efficient eyepiece designs are desired both to decrease cost and improve reliability and mechanical robustness. The head-mounted displays, however, pose two challenges that have made simple eyepieces unsuitable. The first challenge is the need for a large field of view and long eye relief, i.e., distance between the eye and the first optical element. These two opposing requirements place a geometric limitation on the eyepiece design. The second challenge is operation over the full color, red-green-blue visible spectrum. It is well known in the art that it is not possible to provide color aberration correction with only one powered refractive lens material. The result is that most conventional head-mounted display eyepieces have multiple optical elements that must be individually fabricated, aligned, and assembled. This adds significant cost and complexity to the end-product.
One attempt to overcome the chromatic problem is to create an eyepiece that relies upon reflective power, which inherently has no chromatic aberration. The “Solid-Schmidt” design is well known in the prior art, using a powered mirror surface in conjunction with a double-pass beamsplitter surface. See, e.g., U.S. Pat. No. 5,696,521 to Robinson, et. al. However, the Solid-Schmidt's reliance upon reflective optical power makes it less able to image a wide field of view without adding an exterior corrector lens.
Nonetheless, there are eyepiece solutions in the market that achieve good optical imaging performance in a monolithic, single-element package. An example was disclosed in U.S. Pat. No. 5,701,202 by Takahashi. It includes a monolithic element generally made of molded plastic, and makes use of aspheric curvatures for the refractive entrance and exit ports, as well as an asphere on the reflective surface inside the element. The powered reflector has the advantage of minimal color aberration, and thus this eyepiece is well suited for use with color micro-flat-panel display devices on the order of 0.7″-1.0″ diagonals. The coatings on the exit surface nearest to the eye pupil are designed to operate in transmissive mode over a range of angles near normal to the surface, and in a reflective mode for steeper angles coming from the display device. The primary disadvantage with this design form is that it does not provide a simple means for adding either a “see-through” optical path with no optical power, or for adding a second image plane for the purpose of optically combining two image sources located at finite conjugate distances.